A conventional developing device used in an image forming apparatus for developing an electrostatic latent image with developing material such as toner has a storing chamber for storing toner, a waiting chamber for storing toner to be used for developing immediately afterwards, a feeder for feeding toner from the storing chamber into the waiting chamber, and a developing roller positioned inside the waiting chamber and in part opposed to an image bearing member or a photosensitive drum. The developing roller rotates and carries a certain amount of toner necessary for development from the waiting chamber. The toner on the developing roller is supplied to the photosensitive drum for the development of the electrostatic latent image into a visualized toner image.
For the developing roller to stably receive the necessary amount of toner from the waiting chamber, the waiting chamber needs to hold a certain amount of toner. In order for the waiting chamber to hold the certain amount of toner, an amount of toner from the storing chamber to the waiting chamber is controlled by the feeder which is driven on the basis of a signal from a sensor detecting the amount of toner stored in the waiting chamber. Additionally, a developing device disclosed in the Japanese Patent No. 2927656 has an opening, allowing excessive toner to flow freely out of the waiting chamber in order to prevent an occurrence of an excessive toner pressure in the waiting chamber which would hamper the stable rotation of the developing roller.
Another important consideration for the image forming apparatus using such developing device is that the developing roller should be arranged horizontally. Otherwise, the amount of toner calculated by the signal from the sensor such as pressure sensor can differ from that actually stored in the waiting chamber. This may result in that an excessive or insufficient amount of toner is transported from the waiting chamber.